Are amd cards good for other things rather than just gaming? I'm going to be doing gaming, video editing and photoshopping, and I was thinking of getting 2 r9 290s in crossfire, as it's much better than 780s for 4k gaming or triple monitors. My main thing is going to be gaming, but are nvidia cards better for other things? And if so, will it be worth it do drop a few fps in games and pay an extra $200, (total price for both cards) as the 780 is slower than the r9 290?
If you can find the 2 290's, go with them. I am a long time Nvidia user, but the difference in price is not going to gain you anything you would ever notice. Those Radeon cards in Crossfire wil rock. In addition they are the main choice if you ever decide to get involved in mining 'Bit Coin'.
Re. 4K gaming and triple monitors, before you start building you want to study up on the 4K or UHD monitors etc. and how much "horsepower" is actually needed for decent frame rates. I am fairly certain there is some info here on Tom's, and MaximumPC had a very good article on 4K in their April 2014 issue, and some more info in their review of the Dell monitor in the May issue.